Polyester backed shielding tapes are widely used in plenum rated products, such as plenum rated cables. Typically, these polyester backed shielding tapes are used in conjunction with a low cost plenum polyvinyl chloride compound. However, an undesirable synergistic effect often occurs between the polyester in the shielding tape and the polyvinyl chloride compound. As a result, this synergistic effect increases the chances that the construction of the plenum rated product will fail the peak and/or average low smoke and flammability requirements of the Underwriters Laboratories, Inc. (UL) 910 plenum flame test. In addition, the aforementioned synergistic effect increases the chances that the construction of cables with a limited smoke design such as, for example, found in many non-plenum cables, will fail the limited smoke requirements of the UL 1685 fire-propagation and smoke release tests. The requirements of each test are briefly described below.
The UL 910 plenum flame test criteria are as follows. First, the flame spread or propagation must be less than or equal to five (5) feet. Second, peak smoke must be less than or equal to 0.50. Third, average smoke must be less than or equal to 0.15. Likewise, the criteria of the UL 1685 (UL Method & CSA FT4 Method) fire-propagation and smoke-release test is as follows. First, the cable damage height (for cables which have been exposed to a flaming ignition source) is to be less than 8 feet, 0 inches (244 cm) for UL 1685 (UL Method) when measured from the bottom of the cable tray and 150 cm or less for UL 1685 (CSA-FT/4 Method) when measured from the lower edge of the burner face. Second, the total smoke released is to be 95 m2 or less for UL1685 (UL Method) 150 m2 or less for UL 1685 (CSA-FT/4 Method). Third, the peak smoke release rate is to be 0.25 m2/s or less for UL1685 (UL Method) and 0.40 m2/s or less for UL 1685 (CSA-FT/4 Method).
Thus, efforts have been made to comply with the requirements of the UL-910 plenum flame test and the UL 1685 limited smoke criteria to eliminate the above-described smoke problems associated with the use of polyester backed shielding tapes. Specifically, the jacket material in plenum rated cables has been changed from a low temperature thermoplastic material to SOLEF which is a polyvinylidene fluoride based copolymer (PVDF) and a trademark of Solvay Chemicals. The low temperature thermoplastic material and the use of SOLEF in the jacket both protect the inside of the cable which can typically be flame or limited smoke resistant. However, the use of SOLEF in the jacket is preferred over the low temperature thermoplastic material as it generally appears to resolve the aforementioned smoke problems. Thus, while the use of SOLEF as a jacket material appears to resolve the smoke problems, its use significantly increases the cost of the cables.
In addition to the above-mentioned polyester backed shielding tapes and SOLEF, there are other materials which are used in cables. One such material is polypropylene which is typically used as an insulator on individual conductors for non-plenum cables. Aside from its use as an insulator, other uses for polypropylene include polypropylene backed shielding tapes in non-plenum applications. Because polypropylene backed shielding tape can take folds, no compromises in the shielding effectiveness in the cable by increasing the chance of shield shorts have been noted.
TEFLON, a fluoropolymer and a trademark of the DuPont Company, is also used as a conductor insulation and/or in cables but has smoke-related problems similar to those described above and has a relatively high cost factor.
Another material often found in cables is polyethylene. However, because polyethylene, like polypropylene, readily ignites, it does not comply with the UL-910 plenum flame test. Thus, polyethylene is most commonly used in non-plenum applications.
To date, the known polyester backed shielding tapes generally do not comply with the requirements of the UL-910 plenum flame test. The use of SOLEF typically generates prohibitively high manufacturing costs. In addition, TEFLON includes limitations similar to those described in relation to SOLEF whereas, polyethylene readily ignites and cannot be used for plenum application. Thus, economic materials which may be used in plenum rated products and, at the same time, which satisfy the low smoke and flammability requirements and the limited smoke requirements of the UL-910 plenum flame test and the UL 1685 respectively, are needed.